ASTM Designation C162-56 (Standard Definition of Terms Relating to Glass and Glass Products) defines crystal glass as "a colorless glass, highly transparent, frequently used for art or tableware". For centuries it has been thought that the presence of a substantial quantity of lead oxide was required in the composition of the finest crystal glassware. Hence, many of the fine crystal glassware compositions contain 24-30% by weight lead oxide in their formulations, with some glass compositions containing even greater amounts For example, crystal glassware marketed by Corning Incorporated, Corning, N.Y. under the trademark STEUBEN.RTM. contains about 30% by weight PbO. In Europe, lead crystal glassware is required to contain at least 24% lead oxide. It has been conventional wisdom that the inclusion of lead oxide in the base glass formulation imparted such special qualities to the glass as brilliance, clarity, meltability, sonority (resonance), weight, and workability. Stated in another way, glasses demonstrating that combination of properties could not be produced in lead-free glass compositions.
One of the desirable attributes of fine lead crystal glass is the bell-like tone emitted when an article of such glass, such as a dinner plate, goblet, vase, bowl, etc., is struck on the rim thereof with an object such as a pencil or a piece of metal flatware, e.g., a knife, fork, or spoon. In bell technology this sound is characterized by a "strike" note followed by a resonant, prolonged "hum" note. The duration of the hum (or resonance) comprises a significant contribution to the overall beauty of the tone emitted The presence of lead in the glass formulation was assumed to account for the long duration of the resonant hum in fine crystal glass because ware fashioned from common soda-lime-silica glass exhibit very little, if any, resonance.
Another perceived hallmark of fine crystal glass is the weight thereof. In other words, the "heft" of a piece of glass indicates its richness. Therefore, crystal glasses generally have had densities of at least 2.7 grams/cm.sup.3, with many fine lead-containing crystal glasses demonstrating densities of 2.8 grams/cm.sup.3 and higher. STEUBEN.RTM. glassware exhibits a density of about 3 grams/cm.sup.3.
Although fine crystal glass is not required to display the same level of resistance to chemical attack as ware designed for use in scientific laboratories, at the very least it must not develop a hazy surface upon extended exposure to the ambient environment. In certain older lead-containing crystal glasses the surface deteriorated so extensively that it was subject to crazing and/or crizzling.
Yet another feature of fine lead crystal glassware, particularly in those instances where the glass is to be hand worked, as is the case with STEUBEN.RTM. glassware, is the long working range of the glass before it becomes too viscous to shape. If, at that point the piece is not completely formed into a desired shape, it is reheated in a furnace until it again becomes workable and can be further shaped. During this reheating no surface crystallization can be tolerated. The optimum viscosity for shaping the ware is considered to be about 10.sup.5 poises (10.sup.4 Pa.s). For STEUBEN.RTM. glassware the temperature corresponding to that viscosity is about 790.degree. C.
One recognized unfavorable characteristic of fine lead crystal glassware is the ease with which it can be scratched. One measure of the scratch resistance of a glass is the hardness of its surface. And the surface hardness of glasses containing high amounts of lead is invariably low. Moreover, a glass having a harder surface generally exhibits a more brilliant appearance than a glass of lower surface hardness; that brilliance enhancing the overall beauty of the glass.
Where a glass is to be used in tableware applications, it must resist attack by acids and alkalis present in foods with which it may come into contact. The toxicity of lead has been recognized for many years. In recent years there has been a movement to reduce/eliminate lead from all cookware and tableware Thus, the Federal Food and Drug Administration (FDA) has established maxima for lead release which food-contacting surfaces must not exceed, and some state statutes mandate the total absence of lead from food contacting surfaces Those circumstances have given rise to the production of lead-free enamels and glazes for use in coating and decorating food service ware such as are disclosed in U.S. Pat. No. 4,084,976 (Hinton), U.S. Pat. No. 4,224,074 (Nigrin), U.S. Pat. No. 4,282,035 (Nigrin), U.S. Pat. No. 4,285,731 (Nigrin), and U.S. Pat. No. 4,590,171 (Nigrin). The properties of those glasses, however, do not correspond to those demonstrated by fine crystal glass.
Therefore, whereas the release of lead from high quality crystal glass is very low, because of the current impetus to eliminate lead from any ware coming into contact with food, the primary objective of the present invention was to devise glass formulations free from lead, but which would exhibit all of the aesthetic, chemical, and physical properties inherent in fine lead crystal glasses.
Moreover, lead-containing products are considered to constitute hazardous waste materials, therefore requiring special and costly disposal means. While, unlike the disposal of lead-containing batteries and television receiver sets, this is no problem for the individual who owns lead crystal glassware, it is a major concern for the glass manufacturer who must dispose of the dross from the manufacturing process. Accordingly, another advantage flowing from the elimination of lead from the glass composition is the consequence that it minimizes landfill problems.